1. Technical Field
The present invention generally relates to sensors and methods for fabricating sensors. More particularly, the present invention relates to sensors, for example, interferometric sensors, fabricated by photolithographic methods having improved reliability and sensitivity.
2. Background of the Invention
Acoustic emission (AE) monitoring has been proven as a suitable nondestructive technology for structure integrity monitoring, diagnostics, and prognostics, among other things. For example, elastic strain waves generated by rapid release of energy produce AE waves during dislocations in materials. These dislocations can be produced, for example, by fatigue cracks (and their growth), impact, inter-surface slippage, twinning, phase transformations, plastic deformation, and corrosion fatigue.
In order to detect AE activity, AE sensors are typically integrated into the target structure to detect and monitor characteristic signals. Typically, the detected (and, typically, recorded) signals are compared to the theoretical and standard sample signal waveforms. The comparison of the waveforms can be used to determine whether the AE activity detected is from material damage or environmental noise. When a sensor array is used, the location of the AE source can be determined. Typical applications of AE detection include their use on pressure vessels, storage tanks, heat exchangers, piping, reactors, aerial lift devices, and nuclear power plants and equipment, among many other types of structures that can be monitored.
Aircraft fatigue monitoring is a prime example of the use of AE monitoring. For example, critical structures within an aircraft, such as, the connecting lugs between the wings and the main fuselage, can be monitored with an AE sensor to detect fatigue cracking. In such applications, an AE sensor should impose the least impact to the structure's weight, surface shape, and mechanical/chemical properties. Therefore, it is preferred that an AE sensor be compact, lightweight, reliable, sensitive, and have low power consumption.
AE sensing can also be used in partial discharge (PD) acoustic detection in high voltage transformers in the power industry. In this application, the principal considerations for selection of AE sensors are immunity to electromagnetic interference and immunity to chemical erosion.
Typical prior art sensors that are used for AE detection include piezoelectric sensors, Fiber-Bragg-Grating (FBG) sensors, and Fabry-Perot (F-P) optical fiber sensors. Of these types of AE sensors, piezoelectric sensors are most widely used because of their high sensitivity, low cost, and ease of use. However, piezoelectric sensors are characterized by the following disadvantages:                1. Most traditional piezoelectric AE sensing systems are bulky. The piezoelectric disk is typically so brittle that special packaging to prevent breakage is typically required. In addition, since electrical signals cannot be transmitted far away without electrical amplifiers, piezoelectric sensors require electrical connections and associated electrical devices, which greatly increase the size and the difficulty of mounting piezoelectric sensors. Furthermore, the complexity of piezoelectric systems typically decreases the reliability of systems employing these sensors.        2. Piezoelectric AE sensors normally have large contact surfaces. Typically, such sensors are 6.35 mm or larger in diameter. As a consequence, the output signal from a piezoelectric AE sensor comprises the integration of all points within the contact area. This inherently decreases the accuracy of the piezoelectric sensor.        3. Piezoelectric AE sensors are electrical devices and, as such, are also sensitive to electromagnetic noise. Therefore, piezoelectric AE sensors require special signal processing methods to minimize their sensitivity to noise. Moreover, piezoelectric AE sensors are not suitable in some environments, such as, to monitor nuclear power equipment.        4. In addition, piezoelectric AE sensors are limited by the electronic device and the Curie temperature of the piezoelectric components. Piezoelectric AE sensors are not suitable for applications where the environment temperature is over 573 K.        
Optical AE sensors have shown high resolution and accuracy using an interferometric detection technique, such as, in Fabry-Perot (F-P) cavity or Fiber-Bragg-Grating (FBG) AE sensors. The small size and geometrical flexibility of such optical AE sensors make them easy to be mounted in positions close to critical locations, for example, where cracking and damage are expected to initiate, while optical AE sensors typically do not influence the mechanical properties and performance of target structure. Optical connections and non-conducting sensors make the system immune to electromagnetic interference, insensitive to thermal variation, and inert to chemical erosion. Optical AE sensors can transmit a signal faster and farther than electrical devices. Another outstanding advantage of optical AE sensors is their capability of survival in high-pressure and high-temperature cure environments that are common during structure fabrication, system integration, and daily use.
However, FBG-type AE sensors and high finesse F-P-type AE sensors are typically sensitive to the noise from the environment. The spectrum of these optical sensors is so sharp that small deviations of the laser wavelength or small changes in the environment can shift the spectrum greatly. FBG sensors and intrinsic F-P interferometric (IFPI) sensors may drift greatly due to the uncertain polarization state, refractive index variation with temperature, and unreliable bonding points. Currently, the most common prior art solution is to lock the laser wavelength to the center of the optimized modulation position in the reflection spectrum. However, locking the laser wavelength increases the complexity and cost of the optical AE sensor system. This problem becomes intolerable when multiple sensors are used to establish a network, and each of optical AE sensors needs an independent monitoring and tuning system. Another solution is to use a short FBG or a short-cavity-length F-B sensor. Also, the sensing area of an optical AE sensor, such as, the length of the FBG, should be less than the wavelength of the acoustic wave detected. Otherwise, the output from the FBG-type AE sensor will be distorted by the averaging effect on the change of the grating pitch or FBG cavity. However, this typically will decrease the sensitivity of the FBG AE sensor and increase the fabrication difficulty of F-P AE sensor.
Diaphragm-based, extrinsic F-P interferometric (EFPI) optical sensors can avoid the disadvantages mentioned above optical AE sensors. EFPI AE sensors are small and compact in size while maintaining the advantages of the optical fiber sensors at the same time. According to aspects of the present invention, as will be discussed below, a diaphragm of an F-P sensor can be fabricated by MEMS technology, which has high potential for providing low cost, good repeatability, and high yield.
As is known in the art, because acoustic waves from AE are typically from 100 k Hz to 1 MHz, a spectrum demodulation method is typically not fast enough for EFPI AE sensors and an intensity demodulation method is normally used. However, as will be discussed below, aspects of the present invention overcome or minimize this disadvantage of EFPI AE sensors.
Moreover, accurate cavity length control is very important for EFPI AE sensor fabrication and high quality thin diaphragm fabrication for EFPI AE sensors are difficult to achieve with current design and fabrication techniques. For example, although cavity length control of 3 nanometers (nm) precision has been reported, the diaphragm thickness used was about 5 μm is too thick to achieve the high sensitivity desired for AE detection. This undesirable diaphragm thickness limitation and poor repeatability was a result of the fabrication method used.
Though photolithographic methods have been used in the prior art to fabricate diaphragms, the uniformity of the cavity length in the F-P cavity is difficult to control and the yields are poor.
In addition, prior art methods of mounting optical fibers, whose end face serves as one of the reflection surfaces of F-P cavity, are typically bonded by epoxy glues. The use of such glues introduces problems for F-P-type AE sensors, such as, reduced reliability and spectrum shift caused by temperature variation.
U.S. Pat. No. 5,381,231 of Tu; U.S. Pat. No. 5,087,124 of Smith, et al.; and U.S. Patent Publication 2007/000663 of Zerwekh, et al. all disclose interferometric sensors having optical fibers. However, none of these references provide the teachings or advantages of aspects of the present invention.
The prior art methods of fabricating optical AE sensors cannot meet the requirements of high performance, high yield, and low cost at the same time because of the difficulty in controlling cavity length and diaphragm thickness. Prior art methods of fabrication are complex and costly fabrication process. Accordingly, there is a need in the art for method of fabricating an optical AE sensor that provides precise cavity length control, high sensitivity, good thermal stability and repeatability, simple fabrication and packaging process, and high-volume production. Moreover, there is a need in the art for accurate optical AE sensors having high sensitivity, good thermal stability, and good repeatability. Aspects of the present invention address these shortcomings and disadvantages of the prior art.